This paper presents the numerical investigation of the purely oscillatory laminar flows of incompressible Newtonian fluids, in the entrance region of parallel plate microchannels. Development of the axial velocity profiles and the required entrance length were studied in the low Reynolds number regime (20[Formula: see text] 200) and for the low dimensionless oscillation frequency or the Stokes number (1.08 [Formula: see text] 2.80), which is applicable particularly for microchannel flows. To obtain more realistic and applicable results, the time-dependent parabolic entry conditions were considered for all simulations. The results show that for ([Formula: see text]1.53), the entrance length can be estimated by the steady-state results for corresponding inlet velocity profiles, while for the (1.53 [Formula: see text] 2.80), the deviation from the steady-state condition occurs for the entrance length. Further, according to the obtained numerical data, in this work, a useful correlation is proposed to predict the entrance length for the studied ranges of the purely oscillatory flows through the parallel plate microchannels.

An experimental study was performed to investigate the heat transfer characteristics of the convection flow through a rectangular air duct with aspect ratio of 10 (a/b = 10) which is filled with metallic porous materials. All four walls of the duct are subjected to a constant and uniform heat flux. The Reynolds number based on the hydraulic diameter has been kept between 500–2000 in order to ensure the laminar flow through the duct. The effect of different parameters such as variable porosity and density of porous layers have been investigated. For different porous layers configuration, heat flux at the walls, wall temperatures and air mass flow rate has been measured and the Nusselt number has been calculated. The results are compared with the clear flow case where no porous material was used. it can be concluded that higher heat transfer rates can be achieved in porous media flow case at the expense of a reasonable pressure drop. Based on the experimental data new empirical correlations for both Nusselt number and friction factor have also been developed for such air duct, which gives a good agreement between predicted values and experimental values of Nusselt number and friction factor.